It is well known that twines and cords for a variety of purposes may be produced almost entirely from natural fibers. While such natural fiber twines and cords have filled their intended purpose, more recently they are being replaced by superior synthetic products; for example, as evidenced by the rapid increase in usage of polypropylene yarns for rope construction. Such synthetic twines have also found particular application as agricultural twines, for example binder and baler twines, and as commerical twines such as tying twines and cable fillers. In many of these applications, particularly for agricultural and tying twine uses, the synthetic twines are used in connection with automatic tying equipment.
The knot strength of twine, either natural or synthetic, is usually significantly less than the tensile strength of the twine, for example generally on the order of 50% to 60% of the tensile strength. Accordingly, in those applications involving the tying of knots, the knot strength is generally considered as the limiting factor in choosing a particular strength twine. Consequently, in order to achieve a sufficient knot strength for any desired purpose, the tensile strength of the prior art twine generally was much higher than was necessary for a particular application. In other words, since the strength of the twine generally is related to the gauge of the twine, heavy and more bulky twines have been required in order to achieve a desired knot strength.
Some of the known prior art synthetic twines have comprised a plurality of synthetic fibers such as monofilaments, flat tapes, ribbons, etc. which are twisted together, much in the manner that conventional natural fiber twines are twisted together. The twisting operation is for the purpose of containing and holding together the various synthetic fibers to provide a generally unitary structure in which the stress of one fiber is transmitted to the next in order to develop the continuity and strength of the resulting twine and cord. Examples of such prior art twines are known from U.S. Pat. Nos. 3,332,228 (twisting of strips of polypropylene film together to form a twine); 3,422,616 (false twisting of strips of oriented synthetic films to randomly fibrillate the films, and thereafter true twisting a plurality of such films together to form a twine); and 3,402,547 (twisting of films, ribbons, filaments or fibers made from steroregular polypropylene to form a twine). While such prior art twines have exhibited the required strength characteristics, they have also exhibited a tendency to unwind or unravel during use, which, as can be appreciated, is undesirable and tends to decrease the resulting strength and abrasion resistance of the twine during use. Furthermore, such twisted synthetic fiber twines are relatively expensive to manufacture on a commercial bases since it is a two step process requiring specialized equipment; for example, extrusion/orientation of tape yarns followed by a separate and discrete twisting operation.
Another class of prior art synthetic twines have involved the use of a plurality of parallel synthetic monofilaments, generally of a round cross sectional configuration, which are grouped together and then wrapped in a casing or other binding material to maintain a unitary structure so that the stresses on the individual fibers are transmitted to other fibers to develop the required continuity and strength for the resulting twine. Various patents relating to such techniques include, for example, U.S. Pat. Nos. 3,415,919, 3,446,002; and 4,228,641 which disclose the use of a synthetic binder material in thin band form which is spirally wrapped and fused about a plurality of synthetic monofilaments. Such prior art twines are advantageous in the sense that they are continuously produced and eliminate the separate and costly twisting operations. In other words, the individual core monofilaments need not be twisted, such as for example shown in U.S. Pat. Nos. 3,446,002 and 3,415,919, or maybe false twisted (i.e., in which a twist is applied intermediate the ends of the monofilaments and which, if released, would return to a zero twist), such as for example shown in U.S. Pat. No. 4,228,641. With the prior art false twisted twines, the casing or binder material serves to retain a portion of the false twist. While such encased synthetic twines exhibit sufficient strength characteristics, difficulties have been encountered in connection with slippage of knots when such twines are tied. In other words, such monofilament encased twines do not exhibit good cinching properties. Furthermore, the cost of producing such monofilament twines is relatively high since monofilament producing equipment is generally more expensive to buy and operate than equipment for producing sheets of synthetic material which can be slit and oriented to form flat tape yarns.
Therefore, while many of the prior art synthetic twines have proven useful for their intended purposes, i.e., providing desired knotting characteristics, the search has continued for improved twines and methods of manufacturing same which result in a reduction of the cost of the twine and cost involved in the production of such twines. This is particularly true with respect to twines which are used in connection with mechanical and automatic tying equipment, e.g., agricultural twines and commercial twines, wherein the knot strength of the twine is considered to be one of the limiting characteristics in regard to the usefulness of the twine. Accordingly, it is an object of the present invention to provide a synthetic twine having knotting characteristics comparable to those of the prior art but which is more economical to produce, thereby resulting in a less expensive twine.